Preliminary studies of some geothermal areas in India

In order to assess the geothermal resources of the hot springs located in different tectonic regions of India, preliminary geophysical, geochemical and tritium studies were undertaken in Puga valley (Ladakh), Ratnagiri and Kolaba Districts (West Coast) and Bhimbandh (Bihar) areas. The studies indicate that out of the three areas investigated, the Puga valley is the most promising because of its higher geothermal gradient, association of spring waters with magmatic components, its higher estimated reservoir temperature (≥ 200°C) and probable larger available supply of groundwater.     

Radon measurements at three New Zealand geothermal areas

Radon-222 measurements ranging from 0·10 to 62 nCi l⁻¹CO₂ are reported for geothermal areas at Wairakei, Broadlands and Ngawha, New Zealand. They suggest that for Wairakei the origin of the ²²²Rn is deep, follows the carbon dioxide, and there is little contribution from local ground water. The reverse is true at Ngawha, and Broadlands is intermediate. Comparison of radon and carbon dioxide levels gives some information about permeability in a given field. If a negative correlation is found, permeability may be predicted to be low.     

Natural electric field survey in three Southern Italy geothermal areas

The intensity of natural electric fields, as measured in three geothermal areas in southern Italy, varies from a few mV/m to more than 10 mV/m. Such extremely high values may be explained as due to self-potentials generated at the contact between the highly conductive water solutions inside the geothermal system and the waters outside the system and/or at the contact between altered and non-altered rocks.     

The exploitation of hydrogen sulfide for hydrogen production in geothermal areas

Hydrogen is a valuable energy resource and it is widespread in nature. As a matter of fact, researches on hydrogen production are currently experiencing an increasing interest from scientists around the world since this resource is clean and renewable. Several methods of producing hydrogen have been developed in industrialized countries such as the United States of America and Germany.This paper is interested in the process by which hydrogen sulfide of geothermal areas is exploited for hydrogen production. In fact, research advances in this field have concluded that hydrogen sulfide of geothermal resources can contribute significantly and economically in the process of hydrogen generation.The present paper was principally conducted from a literature study and a synthesis of works achieved in recent years in order to highlight the various aspects of hydrogen production from hydrogen sulfide and particularly to study the possibility of the exploitation of Algeria’s thermal resources in this field.     

Cost study of superheating geothermal steam

This study compares the electrical generating costs for hybrid plants, in which fossil fuel superheats geothermal steam, with those for conventional dual-flash plants. Parameters varied in the analysis include the resource temperature and the geothermal gradient. In order to make the results generally useful, we have made conservative estimates of site-dependent variables. The results indicate that superheating can significantly improve the economic competitiveness of geothermal resources that would normally be considered uneconomical. In addition, superheat will increase total plant outputs markedly and enhance thermal efficiencies by 30–40%.     

Evidence for thermal mining in low temperature geothermal areas in Iceland

This study deals with thermal mining in several geothermal systems in Iceland. A number of 2500- to 3000-m deep drillholes have been drilled into low temperature geothermal areas in the country. The conductive gradient outside active geothermal areas has also been mapped, and shows a systematic variation from lower than 50°C/km in the outer parts of the Tertiary basalts to over 100°C/km on the borders of the volcanic zones (rift zones). The difference between formation temperatures inside geothermal systems and the surrounding conductive gradient can be computed as a function of depth. This difference is termed ΔT in this paper. The ΔT-curves show that the upper parts of the geothermal systems are heated and the lower parts are cooled compared to the undisturbed conductive gradient. In many cases the cooling of the lower part is greater than the heating in the upper part, so that a net thermal mining has occurred. This thermal mining is calculated for several geothermal systems, and the systems are compared. The net thermal mining in the top 3000 m appears to be much greater in formations of Pleistocene and Pliocene age. It gradually decreases to zero for formations older than 6 million years. However, the net thermal mining is critically dependent on the maximum depth of water convection in these systems, which is unknown.     

Results of some geothermal studies in Singhbhum Thrust Belt, India

Details of heat flow measurements made at Rakha and Narwapahar along the Singhbhum thrust belt (a Precambrian metamorphic belt in the peninsular shield of India) are reported. Temperatures were measured in a number of boreholes and conductivity data were obtained from about 80 core samples. Heat flow was evaluated taking anisotropy in conductivity into account. Heat flow values of 59 mW/m² and 63 mW/m² obtained at the above two places agree well with the value of 61 mW/m² previously reported for Mosabani. From the present observations, the whole Singhbhum thrust belt appears to be characterized by a uniform heat flow of about 61 mW/m² which is almost the same as the global average but much higher than the mean value quoted for shields. However, the surface heat flow is consistent with the surface radioactivity of the area. A plot of surface heat flow against surface heat generation (from the data on U, Th and K contents of the few samples now available) is well on the heat flow — heat generation line for the eastern United States reported by Roy, Blackwell and Birch (1968), which is considered characteristic of a normal continental heat flow province.     

Revision,calibration, and application of the volume method to evaluate the geothermal potential of some recent volcanic areas of Latium,Italy

The volume method is used to evaluate the productive potential of unexploited and minimally exploited geothermal fields. The distribution of P_CO2 in shallow groundwaters delimits the geothermal fields. This approach is substantiated by the good correspondence between zones of high CO₂ flux, and the areal extension of explored geothermal systems of high enthalpy (Monte Amiata and Latera), medium enthalpy (Torre Alfina) and low enthalpy (Viterbo). Based on the data available for geothermal fields either under exploitation or investigated by long-term production tests, a specific productivity of 40 t h⁻¹ km⁻³ is assumed. The total potential productivity for the recent volcanic areas of Latium is about 28 × 10³ t h⁻¹, with 75% from low-enthalpy geothermal fields, 17% from medium-enthalpy systems, and 8% from high-enthalpy reservoirs. The total extractable thermal power is estimated to be 2220–2920 MW, 49–53% from low-enthalpy geothermal fields, 28–32% from medium-enthalpy systems, and 19–20% from high-enthalpy reservoirs.     

Hg anomalies in soils: A geochemical exploration method for geothermal areas

Hg contents of soils in geothermal areas in the western U.S. were measured and a three-fold distribution was observed: peak, aureole and background. Peak values (up to several 100 ppm Hg) occur in fumaroles of vapour-dominated systems, around hot springs, and in zones overlying steeply dipping, hot-water aquifers. Aureoic values (up to several 100 ppb Hg) are found in zones surrounding the peak areas and delineate areas with shallow geothermal convection. Background values vary between 7 and 40 ppb Hg (geometric mean). Usually, Hg is present in a form that can be easily re-volatilized and released to the atmosphere. Altered areas related to fossil hydrothermal systems can be distinguished from alteration related to active systems by their Hg contents. In the rare cases of Hg enrichments as cinnabar or as traces in other sulphides (pyrite, sphalerite) the Hg is not easily released from its host phase, and distinction between active and fossil systems is not possible. Hg anomaly patterns yield information on the presence as well as the geometry of shallow geothermal circulation patterns. In conjunction with structural geologic data, Hg patterns can be helpful in defining reservoir boundaries and can aid in the selection of drill site location.     

Hydrogeological and geochemical studies of the Efteni and Derdin geothermal areas,Turkey

The Efteni and Derdin geothermal areas are located in northwestern Turkey. Relatively low-temperature springs emerge from the Duzce Fault, a normal-component-dominated fault segment of the North Anatolian Fault System. The thermal waters of the Efteni and Derdin Springs show distinct geochemical and isotopic characteristics since they originate from different geothermal reservoirs and reflect the effects of different water–rock interaction processes. Geothermometry revealed higher reservoir temperatures for the Efteni system, however a strong δ¹⁸O shift, interpreted as being the result of isotopic exchange at high temperatures, was observed in the Derdin system. Hydrogeological and geochemical techniques are applied to identify recharge mechanisms, water–rock interaction processes and to construct conceptual models of these geothermal systems.     

Geochemical exploration of the three most significant geothermal areas of Lesbos Island, Greece

Lesbos Island has several thermal manifestations linked to extensional active faults that act as channels for the ascent of deep thermal fluids.The present work describes detailed geochemical exploration aimed at evaluating the potential of the Lesbos Island geothermal resource. Exploration was carried out on the three sites (Kalloni-Stipsi, Petra-Argenos and Polichnitos) that have the most favourable hydrogeological and structural setting on the island.Hydrogeochemical data reveal the presence between Kalloni and Stipsi of a shallow thermal aquifer with temperatures below boiling point, which coincides with carbon dioxide and temperature anomalies in the overlying groundwaters. All the thermal waters in the study area have fairly similar physico-chemical features; their geochemical temperature is in the range 115–125°C. While low-medium enthalpy geothermal resources are relatively abundant in the three selected areas of Lesbos Island, the presence of a high enthalpy geothermal system is still the subject of debate.     

中低温地热发电系统的研究

针对中低温地热资源和工业余热资源,提出了一种以氨水混合溶液作为工质的动力循环系统。该系统可利用300～500 K的中低温地热资源,基础循环工质为50%和60%的氨水溶液。模拟计算表明,该循环系统的理论热效率为10.4%,热力学完善度为73.4%。文章还进一步分析了汽轮机背压和热源温度对循环系统热效率的影响。该研究为有效利用中低温余热资源提供了借鉴和参考。     

Geochemical monitoring of the Krafla and Námafjall geothermal areas, N-Iceland

The Krafla and Námafjall high-temperature geothermal areas in N-Iceland have been exploited for steam production since the late and early 1970s, respectively. Power generation at Krafla was 30 MW until 1998, when it was increased to 60 MW. At Námafjall the steam has been utilized for operating a 3 MW back-pressure turbine unit, drying of diatomaceous earth and heating of fresh water for space heating. A total of 34 wells have been drilled at Krafla, of which 18 are producing at present. At Námafjall 12 wells have been drilled but only three are productive. The highest temperatures recorded downhole are 320 and 350 °C at Námafjall and Krafla, respectively. Geochemical monitoring in the two fields during the last 20–25 years has revealed decreases in the Cl concentrations in the water discharged from most of the wells that have been producing for more than 10 years. The cause is enhanced colder water recharge into the producing aquifers of these wells due to depressurization by fluid withdrawal from the geothermal reservoir. Such recharge is particularly pronounced in the central part of the Leirbotnar wellfield at Krafla but it is also extensive in the only producing well in the Hvíthólar wellfield. At Námafjall incursion of cold groundwater into the reservoir was particularly intense subsequent to the volcanic-rifting event in the area in 1977. Solute (quartz, Na/K, Na/K/Ca) geothermometry temperatures have decreased significantly in those wells where Cl concentrations have decreased but only to a limited extent in those wells which have remained constant in Cl. This indicates that the changes in the concentrations of the reactive components, on which these geothermometers are based, is largely the consequence of colder water recharge and not partial re-equilibration in the depressurization zone around wells where cooling of the fluid occurs in response to extensive boiling. Aqueous SO₄ concentrations increase as Cl concentrations decrease. Except for the hottest wells, which are low in SO₄, sulphate concentrations are controlled by anhydrite solubility. Increase in SO₄ concentrations is a reflection of cooling as anhydrite has retrograde solubility with respect to temperature. H₂S-temperatures are similar to the solute geothermometry temperatures for wells with a single feed. They are, on the other hand, higher, for wells with multiple feeds, if the feed zones have significantly different temperatures. H₂-temperatures are anomalously high for most wells due to the presence of equilibrium steam in the producing aquifers. The equilibrium steam fraction amounts to 0–2.2% by wt. of the aquifer fluid (0–47% by volume). CO₂ temperatures are anomalously high for some Krafla wells due to high flux of CO₂ from the magma intruded into the roots of the geothermal system during the 1975–1984 volcanic-rifting episode. During the early phase of this episode the Leirbotnar wells were the ones most affected. The new magma gas flux has migrated eastwards with time. Today some wells in the Sudurhlídar wellfield are the ones most affected whereas the Leirbotnar wells have recovered partly or fully. The depth level of producing aquifers in individual wells at Krafla and Námafjall has been evaluated by combining data on temperature and pressure logging and geothermometry results. The majority of wells at Krafla receive fluid from a single aquifer, or from 2–3 aquifers having similar temperature. The same applies to two of the three productive wells at Námafjall.     

Advances in the study of solids deposition in geothermal systems

The closing session of the International Workshop on the Deposition of Solids in Geothermal Systems consisted of the formation of special interest working groups to discuss future research directions in the fields of carbonate deposition, silica scale, sulfide deposition, treatment methodologies, heat exchanger engineering, and water/rock reactions. Recommendations made by the working groups for future research included the following: development of more complete and accurate models of the physical and chemical mechanisms associated with the nucleation, adhesion, and growth of scale minerals onto substrate surfaces; investigation and synthesis of more efficient and cost effective chemical scale inhibitors; improvements in heat exchanger materials, design, and operational characteristics; and development of more general economic models for heat extraction facilities that will allow an evaluation of the costs and benefits of various scale-control strategies.     

Application of geothermal absorption air-conditioning system: A case study

Possible applications of geothermal energy are increasingly attracting worldwide interest as a low carbon source of energy. Sedimentary basins offer an accessible low grade geothermal heat source to launch energetically efficient air-conditioning systems. The natural temperature, porosity and permeability of these sedimentary basins may be sufficient to provide usable geothermal power for heat-driven sorption air-conditioning system. In the Perth metropolitan region, hot sedimentary aquifers with temperatures between 70 and 100 °C are generally available at around 3 km depth or shallower. This paper outlines the possible application of a large scale geothermal absorption air-conditioning system to provide baseload cooling to the main campus of the University of Western Australia (UWA). The conceptual design of this system is proposed. The design considerations and the performance outcomes are presented. The economic analysis based on the real-time cooling load profile at the UWA and the chiller operation scheme demonstrates the viablity of geothermal absorption air-conditioning system. This paper will help engineers appreciate the opportunities and barriers to geothermal applications, the latest developments and how this technology may help to significantly reduce carbon dioxide emissions.     

GIS-supported mapping of shallow geothermal potential of representative areas in south-western Germany—Possibilities and limitations

The increasing interest in the utilization of shallow geothermal energy comes with the wish for better knowledge about the factors influencing its efficiency. For this purposes we focused on the subsurface condition's influence on the use of borehole heat exchangers (BHEs) coupled with a heat pump for heating purposes, since this is the most popular heat-extracting technique. We created maps showing the potential for this technique provided by the thermal underground properties. Therefore, we established an underground model for two study areas in south-western Germany with different geological settings using a geographic information system (GIS). The subsurface has been divided into layers with similar thermal properties based on geological, hydrogeological and lithological information. The layers have been attached with specific heat extraction values, according to the German VDI guideline 4640. For depths of 50 and 100 m, the weighted mean value of the specific heat extraction was calculated and presented within maps. These maps point out how the heat-extraction potential differs within and between the study areas and how it depends on depth.     

Chemical study of geothermal waters of Central Tibet (China)

Thirteen of about 300 areas of thermal springs in Tibet have been studied in detail. The springs fall into four groups, the most important and interesting of which produces silica, boron, lithium and cesium-rich waters. Chemical geothermometers agree within a few degrees in the range 200–220°C. A second group includes waters of the same type, but secondary reactions decrease the silica concentrations and increase the Ca and Mg contents. The third and fourth groups are similar to Pyrenean alkaline waters and to CO₂-rich waters from Massif Central respectively; deep temperatures are low to moderate.     

Locating interesting geothermal areas in the Tuscany Region (Italy) by geochemical and isotopic methods

In the western and central part of Siena province some zones have been individuated for the first time as being of considerable interest to geothermal fluid research.Geochemical prospecting on the thermal manifestations in this area has permitted the delimitation of the zones where the Na-K-Ca ratio reveals high underground temperatures. On the other hand isotopic analyses show that, in these same localities, there is an isotopic exchange between the oxygen of the water and that of the rocks. This exchange has previously been noted in geothermal fluids.These observations lead to the conclusion that the thermal manifestations under study, which also seem to be situated where there are important basement fractures, are the result of a mixing of geothermal steam condensate and the shallow circulation waters.     

Partitioning of rare earths and some major elements in the Kizildere geothermal field, Turkey

Rare earth elements and yttrium (REY), Na⁺, K⁺, Cl⁻, and Ca²⁺ were determined in water, steam, mineral scale and rock samples from the Kizildere geothermal field, Turkey. The CO₂-rich parent fluid originates from a sequence of mica schists with marble intercalations. The chemistry of the parent fluid varies with location and time. The average REY composition of the fluids is derived by extrapolation to the lowest Ca concentrations. The apparent vapor–liquid partitioning factors for REY at 145 °C and 5 bar total absolute pressure are about 0.2, whereas for Ca²⁺, Na⁺, K⁺ and Cl⁻ they are <0.05, about 0.0005, 0.0005 and about 0.02, respectively. Apparent scale-liquid distribution coefficients for REY at 145 and 190 °C are about 0.15 and 0.55, whereas at 100 °C they increase from 0.3 (La) to 1.5 (Lu).